1. Field of the Invention
This invention relates broadly to particle accelerators and specifically to particle accelerators (such as pulsed neutron generators, x-ray sources and gamma ray sources) used in the oilfield industry. More particularly, this invention relates to a high voltage power supply for a particle accelerator that has an intended use in boreholes particularly at elevated temperatures.
2. State of the Art
Pulsed neutron generators are well known in the art. Typically, a pulsed neutron generator (PNG) is an electronic radiation generator that operates at high voltages. The PNG typically incorporates a neutron tube (commonly referred to as a “Minitron”) that produces neutrons by fusing together hydrogen isotopes. More particularly, an ion beam of deuterium or tritium ions are typically accelerated into a metal hydride target that contains deuterium and/or tritium. Fusion of deuterium atoms (D+D) at the target results in the formation of a 3He ion and a neutron with a kinetic energy of approximately 2.4 MeV. Fusion of a deuterium atom and a tritium atom (D+T) at the target results in the formation of a 4He ion and a neutron with a kinetic energy of approximately 14.1 MeV. Fusion of tritium atoms (T+T) at the target results in the formation of a 4He ion and two neutrons with a kinetic energy within a range from 2 MeV to 10 MeV.
The neutron tube typically has several components including:                a gas reservoir (e.g., a filament or hydrogen-gettering material made of metal hydride) to supply reacting gas molecules (such as deuterium and/or tritium);        an ion source that strips electrons from the gas molecules thus generating a plasma of electrons and positively charged ions; these ions are extracted from the plasma so as to form an ion beam;        a target with reacting gas molecules stored in a metal hydride layer; and        an accelerating gap that propels the ions of the ion beam to the target with sufficient energy to cause the desired fusion reaction.All of these components are supported within a vacuum tight enclosure realized by glass and/or ceramic insulators, fused or brazed to metal washers and plates.        
Ordinarily, a plasma of positively charged ions and electrons is produced by energetic collisions of electrons and neutral gas molecules within the ion source. Two types of ion sources are typically used in neutron generators for well logging tools: a cold cathode (a.k.a. Penning) ion source, and a hot (a.k.a. thermionic) cathode ion source. These ion sources employ anode and cathode electrodes of different potential that contribute to plasma production by accelerating electrons to energy higher than the ionization potential of the gas. Collisions of those energetic electrons with gas molecules produce additional electrons and ions. Other suitable ion sources can also be used.
Penning ion sources increase collision efficiency by lengthening the distance that the electrons travel within the ion source before they are neutralized by striking a positive electrode. The electron path length is increased by establishing a magnetic field which is perpendicular to the electric field within the ion source. The combined magnetic and electrical fields cause the electrons to describe a helical path within the ion source which substantially increases the distance traveled by the electrons within the ion source and thus enhances the collision probability and therefore the ionization and dissociation efficiency of the device. Examples of neutron generators including Penning ion sources used in logging tools are described in U.S. Pat. No. 3,546,512 or 3,756,682 both assigned to Schlumberger Technology Corporation.
Hot cathode ion sources comprise a cathode realized from a material that emits electrons when heated. An extracting electrode (also called a focusing electrode) extracts ions from the plasma and focuses such ions so as to form an ion beam. An example of a neutron generator including a hot cathode ion source used in a logging tool is described e.g. in U.S. Pat. No. 5,293,410, assigned to Schlumberger Technology Corporation.
During operation, high voltage power supply circuitry provides a negative high voltage signal to the target such that the target floats at a voltage potential typically on the order of −70 kV to −160 kV DC. The gas reservoir is controlled to adjust the gas pressure within the neutron tube as desired. The gas pressure is adjusted by the heating power levels supplied to the filament or gotten by a gas reservoir. A pulsed-mode ion source power supply circuit supplies pulsed-mode power supply signals around ground potential (for example, pulses on the order of 200V) to the ion source such that ion source produces a pulsed-mode ion beam that is accelerated by the DC electric field gradient in the accelerating gap between the extraction electrode and the target. The electric field gradient is adapted to provide enough energy that the bombarding ions at the target generate and emit neutrons therefrom. Pulse-width modulation of the power supply signals provided to ion source can be used to control the power of the ion beam and therefore the neutron output as desired.
A suppressor electrode shrouding the target can be provided within a vacuum tight enclosure. The suppressor electrode acts to prevent electrons from being extracted from the target upon ion bombardment (these extracted electrons are commonly referred to as secondary emission electrons). To do so, a negative voltage potential difference is provided between the suppressor electrode and the target of a magnitude typically between 200V and 1000V.
The vacuum tight enclosure and the high voltage power supply circuitry are surrounded by high voltage electrical insulating material, and the resulting structure is enclosed in a hermetically-sealed metal housing. The housing is typically filled with a dielectric media (e.g., SF6 gas) to insulate the high voltage elements of the electronics and neutron tube. External power supply circuitry supplies power supply signals via electrical feedthroughs to the high voltage electronics as well as to the gas reservoir and ion source as needed.
During operation, the reaction of the ion beam at the target produces heat thereon. The high voltage insulating materials of the neutron tube that surrounds the target typically have poor thermal conductivity. Consequently, operation of the neutron tube can result in a heat build at the target, which can cause significant degradation of neutron output.